1. Field of the Invention
The present invention generally relates to the field of implantable heart stimulation devices, such as pacemakers. More specifically, the present invention relates to a biventricular heart stimulator for stimulating both ventricles of a human heart and a method for controlling such a stimulator.
2. Description of the Prior Art
In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulses arising from the sinus node are transmitted to the two atrial chambers, causing depolarizations known as P-waves, which result in atrial chamber contractions. The excitation pulses are further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system, causing depolarizations known as R-waves which result in ventricular chamber contractions. An R-wave is also referred to as a QRS complex.
Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient's heart at a prescribed rate and/or rhythm when, through disease or other causes, the heart is not able to maintain the prescribed heart rate or rhythm on its own. When the delivered electrical stimulation pulses are of sufficient energy, they cause the cardiac muscle tissue to depolarize, and therefore contract, thereby forcing the heart rate or rhythm to track the delivery of the stimulation pulses. When the delivered stimulation pulses are of insufficient energy, depolarization does not occur, and the heart rate or rhythm is not controlled by the pacemaker. Hence, for the pacemaker to perform its intended function, it is critically important that the delivered electrical stimulation pulses be of sufficient energy to depolarize the cardiac tissue.
The depolarization and ensuing contraction of the heart in response to a delivered cardiac stimulation pulse is generally referred to in the art as “capture”. Consequently, the term “non-capture” denotes the condition when a delivered stimulation pulse does not result in depolarization and contraction. When detecting capture, sensing circuitry checks for the depolarization of a cardiac chamber following and in response to a delivered stimulation pulse. Such a depolarization as a result of a delivered stimulation pulse is also referred to as an “evoked response” (ER) of that chamber. Furthermore, evoked response is detected during a selected time period following the delivery of a stimulation pulse. Such a time period is generally referred to as an “evoked response window”.
The energy of the electrical stimulation pulses generated by an implanted pacemaker is derived from the energy stored in the pacemaker power source or battery. The pacemaker battery has a limited amount of energy stored therein, and the generation of stimulation pulses represents by far the greatest drain of such energy.
The amount of energy needed to effectuate capture is known as the capture “threshold”. Hence, stimulation pulses of energy less than the capture threshold do not bring about capture, while stimulation pulses of energy greater than the capture threshold do bring about capture. By adjusting the energy of the electrical stimuli so that it is always greater than the capture threshold, but not too much greater, the limited energy of the pacemaker battery may thus be preserved. The battery energy is preserved for two reasons. Firstly, pulses having an energy content insufficient to cause capture, i.e. stimulation pulses below threshold level, are rarely generated. Such pulses represent wasted energy. Secondly, pulses having an excessive energy content, i.e. an energy content greatly exceeding the capture threshold, are also rarely generated. Such excess energy does not only represent wasted energy, but also energy that may disadvantageously cause pectoral stimulation and/or sensation.
Generally, a capture threshold search is performed at predetermined or preprogrammed intervals. A capture threshold search would begin at a desired starting point (either a high energy level or the level at which capture is currently occurring) and decrease the energy level until capture is lost. The value at which capture is lost is known as the capture threshold. Thereafter, in order to secure capture, a safety margin is added to the capture threshold to arrive at the energy content of the stimulation pulse. One of the key issues is to choose the safety margin such that it provides capture, while at the same time provides adequate energy savings and does not cause pectoral stimulation and/or sensation.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle, via either a unipolar or bipolar lead. Single-chamber pacemakers can operate in either a triggered mode or a demand mode. When operating in a demand mode, sensing and detection circuitry allow for the pacemaker to detect if an intrinsic cardiac depolarization, either an R-wave or a P-wave, has occurred within the defined timeout interval. If an intrinsic depolarization is not detected, a pacing pulse is delivered at the end of the time-out interval. However, if an intrinsic depolarization is detected, the pacing pulse output is inhibited to allow the natural heart rhythm to preside.
Biventricular pacemakers are now available and can provide either demand or trigger type pacing in both the right and the left ventricular chambers. In biventricular pacing, one bipolar lead is typically placed in the coronary sinus for pacing and sensing in the left ventricle. Another bipolar lead is positioned in the right ventricle for pacing and sensing in the right ventricle.
Generally, capture verification occurs on a beat-by-beat basis. If no capture is verified, i.e. a loss of capture is detected, the pacemaker provides a backup pulse with an increased energy content. If successive losses of capture are detected, this indicates that an increase in capture threshold has occurred. Then, the pacemaker responds by performing a threshold search, and sets the energy level of the successive stimulation pulses to the new capture threshold plus the added safety margin.
In a cardiac stimulation device arranged for biventricular stimulation, the stimulation threshold and the evoked response are measured both in the first and the second stimulated ventricle. However, a problem with known biventricular pacemakers equipped with features to assure capture after stimulation pulses, is that it is difficult to verify capture on every beat, particularly if the pacemaker operates with a delay between the first stimulation pulse delivered to the first ventricle and the last stimulation pulse delivered to the second ventricle. Such a delay is known as an interventricular delay or interval, or a VV delay, and is generally provided in order to control the contractions of the ventricles in desired manner.
After a delivered stimulation pulse, the evoked response detection window typically ends 50-100 ms after the stimulation. Thus, an interventricular delay chosen such that the stimulation pulse is delivered to the second ventricle during this period, will result in an interference with the detection of the evoked response resulting from a delivered stimulation pulse in the first ventricle. In other words, it would be difficult to verify loss of capture.
One attempt to address this problem has been to only verify capture or loss of capture in the second ventricle on a beat-by-beat basis. Thus, capture is not usually verified on a beat-by-beat basis in the first ventricle, but rather after certain programmable time intervals, for instance every 15 minutes or 1 000 heartbeats. At these instants, the evoked response (ER) window for the first ventricle is normally made clear from disturbing stimulation pulses in other places of the heart by a temporarily changed timing pattern for the delivery of stimulation pulses. For instance, the order of delivering stimulation pulses to the first and second ventricle could be shifted, the stimulation pulses could be simultaneous, or the time interval between the pulses to first and second ventricle, known as the interventricular delay, could be adjusted. When losses of capture is detected in the first ventricle, a capture threshold search is performed in order to adjust the stimulation level, if necessary, to the changed capture threshold.
To ensure capture between these capture verifications for the first ventricle, a fixed safety margin is introduced. This safety margin for the first ventricle is normally higher than the safety margin in the second ventricle to account for the fact that the capture verification is not performed on a beat-by-beat basis in the first ventricle.
Changes in capture threshold is normally very slow and the increased safety margin for the first ventricle is normally sufficient to avoid loss of capture in spite of the time period provided between successive capture verifications for the first ventricle. However, studies have shown that during rapid changes in the capture threshold, time periods without capture occur for the first ventricle. If this happens, there is a chance that the patient will not receive the intended cardiac therapy, which in turn can impair the ability of the patient to perform work and deteriorate the state of the heart disease. Such rapid changes could for instance be due to infections, metabolic changes, or medical drugs.